Overcoming Trifluralin Antagonism of Glyphosate

Eleventh Australian Weeds Conference Proceedings

OVERCOMING TRIFLURALIN ANTAGONISM OF GLYPHOSATE

Wendy Bedggood Daratech Pty. Ltd., Victorian Institute for Dryland Agriculture, Private Bag 260, Horsham, Victoria 3400, Australia

Summary Investigations into the cause of loss of A series of experiments were conducted to isolate the glyphosate activity were carried out after reports of factors effecting antagonism and to find a means of over- reduced glyphosate activity in trifluralin glyphosate tank coming trifluralin antagonism of glyphosate. mixes used in the 1995 cropping season. It was found that the antagonism increases as the ratio of trifluralin to MATERIALS AND METHODS glyphosate increases and at a ratio of 2:1 significant loss A series of bioassays was conducted to compare the phy- of glyphosate activity occurred. The antagonism was totoxicity of various tank mixes of trifluralin and eliminated by the addition of ammonium sulphate at 2% glyphosate. First an experiment assessed the effect of of spray solution. three ratios of trifluralin to glyphosate on activity of glyphosate. A second experiment assessed four spray ad- INTRODUCTION ditives for their ability to reduce antagonism. A final In the 1995 cropping season wet weather delayed the bioassay verified the results with dose response curves to sowing of winter crops and limited paddock preparation measure the reduction of antagonism by ammonium sul- time. As a result there was insufficient time to apply pre- phate. and post-emergent herbicides in separate operations. Consequently an off label tank mix of trifluralin and Plants Oats (Avena sativa L. cv. Echidna) were grown in glyphosate was used widely in the Wimmera. Some her- Debco® potting mix and supplied with adequate nutri- bicide sales representatives estimated that 30% of the ents (1 plant per 10 cm diameter pot). Plants were grown trifluralin sold in the Wimmera last year was used in this in the glasshouse until they reached the 1.5 to 2-leaf stage tank mix. Most farmers were satisfied with the results (14 days). Uniform plants, one plant per replicate, were from this tank mix, however there were some reports of then selected for herbicide treatment, seven replicates reduced glyphosate activity. per application rate. After herbicide application all plants It is well documented that glyphosate is commonly were returned to the glasshouse and arranged in antagonized by cations and other herbicides. Nalewaja randomized blocks. Glyphosate activity was assessed by and Matysiak (1991) reported cation antagonism of measuring plant fresh weight 14 days after herbicide ap- glyphosate occurred with iron, zinc, calcium, magne- plication. sium, sodium and potassium. In contrast sulphate, phosphate, citrate and acetate anions were not antagonis- Herbicide application Herbicide was applied to plants tic and that the anions of ammonium compounds were of using a laboratory track sprayer delivering 64 L ha-1 at primary importance in overcoming glyphosate antagonis- 200 kPa via three Spraying Systems 11001 flat fan tic salts. Flint and Barrett (1989) reported antagonism of type nozzles. Nozzles were spaced 50 cm apart and 35 glyphosate by four forms of 2,4-D and two forms of cm above the plants. The boom traversed the plants at dicamba, antagonism was more significant where 6 km h-1. glyphosate was at the lower rates and 2,4-D and dicamba were at the higher rates. Hydrick and Shaw (1994) inves- Ratio experiment In the first experiment treatments tigated the effects of tank mix combinations of were trifluralin plus glyphosate as tank mix ratios of 0:1, non-selective foliar and selective soil-applied herbicides 1:2, 1:1 and 2:1. The trifluralin used was an EC (400 g on three weed species and reported antagonism was the a.i. L-1) and the glyphosate was the isopropylamine salt most frequent interaction and usually occurred when the (450 g a.i. L-1). Rates for glyphosate were 66, 105, 150, lower rates of non-selective foliar herbicides were used. 200, 300, 450, 600 and 1000 g a.i. ha-1. All treatments When the rates of non-selective herbicide were increased had Chemwet 1000® added to the tank at 0.2% spray vol- antagonism was usually overcome. Wills and McWhorter ume. (1995) reported salts of the monovalent cations ammonium, potassium, sodium and to a lesser degree, the anion Spray additive experiment In a second experiment four phosphate, generally resulted in increased toxicity of spray additives (Table 1) were added to the tank mix to glyphosate. test their ability to eliminate antagonism. Results from

184 Eleventh Australian Weeds Conference Proceedings the first experiment showed the greatest antagonism at a tank mix ratio of two trifluralin to one glyphosate, there- 120 fore this ratio was used in this experiment. Two tank mix 100 rates were used: trifluralin at 120 and 600 g a.i. ha-1 with 80 -1 glyphosate at 60 and 300 g a.i. ha respectively. All treat- 60 ® ments had Chemwet 1000 added at 0.2%. 40

ED 50 (g glyphosate/ha)ED 20 Experiment to measure effectiveness of Boost® A third experiment established dose response curves for 0 glyphosate, trifluralin plus glyphosate and trifluralin plus 0:1 1:2 1:1 2:1 trifluralin to glyphosate ratio glyphosate plus ammonium sulphate. Again a tank mix ratio of 2:1 trifluralin to glyphosate was used with Figure 1. Activity of glyphosate on oats applied alone glyphosate rates at 40, 60, 80, 100, 200, 400, and 800. and when tank mixed with trifluralin at 1:2, 1:1 and 2:1 Boost was added at 2% of spray volume. Chemwet 1000® glyphosate:trifluralin (error bars = 95% least significant was added to all treatments at 0.2% spray volume. intervals).

Statistical analysis Non-linear regression analysis was These findings may explain why only some farmers used to fit logistic dose response curve to the data in ex- reported loss of glyphosate activity. Growers preparing periment 1 and 3 . The ED50 was calculated with a 95% paddocks for canola or legumes could have used up to least significant interval for comparison between treat- 945 g ha-1 trifluralin in the tank mix. Mixing this with the ments. highest reccommended label rate of glyphosate 450 g ha-1 gives a ratio of 2:1. If lower rates of glyphosate were RESULTS AND DISCUSSION used, the ratio would increase resulting in greater antago- Ratio experiment It was demonstrated that as the pro- nism and less activity. portion of trifluralin was increased in the tank mixture the activity of glyphosate decreased. This concurs with Spray additive experiment The addition of ammonium the findings of Hydrick and Shaw (1994) when mixing sulphate to the trifluralin glyphosate tank mix appeared imazaquin and metribuzin with glyphosate, as the pro- to eliminate the antagonism. Glyphosate applied alone portion of these chemicals increased the activity of reduced the fresh weight of oats by 96% (Table 2) when glyphosate decreased. compared to the unsprayed controls. The addition of The efficacy of glyphosate was significantly reduced trifluralin to glyphosate at a 2:1 ratio reduced the fresh in glasshouse trials when trifluralin was added to the weight of oats by 52% of the unsprayed controls. When spray tank mixture at a ratio of 1:1 or 2:1 trifluralin to ammonium sulphate was added to the tank mix the fresh glyphosate on active ingredient. The tank mix ratio of 2:1 weight of oats was reduced to 96% of the unsprayed con- reduced glyphosate activity by 46%. The 1:2 ratio treat- trols indicating a complete elimination of antagonism. ment was not significantly different from glyphosate Potassium dihydrogen orthophosphate at either concen- sprayed alone (Figure 1). tration, Flowright® and Li-700® added to the tank mix reduced oat fresh weight by 71, 37 and 47% respectively Table 1. Additives used in experiment and amounts when compared to the unsprayed controls. used for each treatment.

Additive Amount Table 2. Oat fresh weight expressed as a percentage re- KH2PO4 2 parts to 1 glyphosate duction of controls. KH2PO4 4 parts to 1 glyphosate Treatment % reduction oat fresh Li-700® 345 g L-1 soyal 0.2% of spray water weight -1 phospholipids 355 g L Glyphosate 96 proponoic acid Tankmix (TM) 52 ® -1 Boost 500 g L 2% of spray water TM and KH2PO4 low rate 71 ammonium sulphate TM and KH2PO4 high rate 71 Flowright® 920 g L-1 1 part trifluralin to 3 parts TM and Li-700® 47 blend of fatty Flowright mixed together TM and Flowright® 37 triglycerides before adding to tank. TM and Boost ® 96

185 Eleventh Australian Weeds Conference Proceedings

The results presented in Table 2 are treatments REFERENCES sprayed at 600 g trifluralin plus 300 g glyphosate per hec- Flint, J.L. and Barrett, M. (1989). Antagonism of tare. The lower tank mix rate treatments showed a glyphosate toxicity to Johnsongrass (Sorghum similar trend. halapensis) by 2,4-D and dicamba. Weed Science 37, 700-05. Effectiveness of ammonium sulphate experiment The Hydrick, D.E. and Shaw, D.R. (1994). Effects of tank- third experiment was carried out to verify the results ob- mix combinations of non-selective foliar and tained in the previous experiment. A complete dose re- selective soil-applied herbicides on three weed sponse curve was obtained for each treatment and the species. Weed Technology 8, 29-133. ED50 calculated for each curve. Nalewaja, J.D. and Matysiak, R. (1991). Salt antagonism The ED50 for glyphosate sprayed alone was 57 g ha-1 of glyphosate. Weed Science 39, 622-8 (Figure 3). which was significantly lower than the ED50 Nalewaja, J.D. and Matysiak, R. (1992). Species differ for trifluralin:glyphosate at 2:1 ratio which was 150 in response to adjuvants with glyphosate. Weed g ha-1. Trifluralin:glyphosate at a 2:1 ratio with the addi- Technology 6, 561-6. tion of ammonium sulphate at 2% had an ED50 of 35 g Wills, G.D. and McWhorter, C.G. (1984). Effect of ha -1 which was significantly lower than both other treat- inorganic salts on the toxicity and translocation of ments. glyphosate and MSMA in purple nutsedge (Cyperus rotundus). Weed Science 33, 755-61. 160 140 120 100 80 60 40

ED50 (gED50 glyphosate/ha) 20 0 glyph glyph/trif glyph/trif/boost Figure 3. Activity of glyphosate on oats alone, tank mixed with trifluralin and the addition of ammonium sulphate to the tank mix (error bars = 95% least significant intervals).

It is concluded that ammonium sulphate effectively DISCLAIMER overcame trifluralin antagonism of glyphosate toxicity to The information contained in this paper is offered by the oats. However, Nalewaja and Matysiak (1992) reported State of Victoria through its Department of Natural Re- that the effectiveness of diammonium sulphate in over- sources and Environment solely to provide information. coming calcium antagonism varied with species. While the information contained in this paper has been Therefore further investigation would need to be carried formulated with all due care by the Department of Natu- out across a broader range of species before ammonium ral Resources and Environment, the State of Victoria its sulphate could be recommended to overcome antagonism servants and agents accept no responsibility for any per- in this tank mix. Such a recommendation would be use- son acting or relying on the information contained in this ful in situations where canola and legume crop paper and disclaims all liability for any error, omission, preparation are planned using tank mixes of trifluralin loss or other consequence which may arise from any per- using low rates of glyphosate. son relying on anything contained in this paper.

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